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Oct. 17, 2011 -- Low-birth-weight babies may be five times more likely to be later diagnosed with autism than children born at a normal weight, according to a new study.

Researchers say it's the first study that followed children over time to confirm the link between low birth weight and diagnosis of autism spectrum disorder (ASD) in a large group of infants followed for 21 years.

The results showed 5% of low-birth-weight babies were eventually diagnosed with autism spectrum disorder compared with an autism rate of about 1% in normal-weight babies.

"We anticipated that there would be a higher rate among this premature group," says researcher Jennifer Pinto-Martin, MPH, PhD, director of the Pennsylvania Center for Autism and Developmental Disabilities Research and Epidemiology. "We didn't anticipate that it would be five times higher than the general population."

Pinto-Martin says the results aren't cause for alarm, but rather a call for vigilance. She says children who are born at low birth weight should be monitored and screened early and often for autism so they can benefit from early diagnosis and treatment.

"We really can do a lot with a young child," Pinto-Martin tells WebMD. "Their brain is highly malleable and really ready for intervention, and the older they get the harder it gets."

Low Birth Weight and Autism Risk

Low birth weight and premature birth are known risk factors for mental and movement disabilities. But researchers say the relationship between these risk factors and autism isn't well understood.

In this study, published in Pediatrics, researchers followed 623 children born at low birth weight (about 1 pound to about 4.4 pounds) between 1984 and 1989 in New Jersey.

All of the children were screened for autism spectrum disorder at age 16. This initial autism screening showed that 117 children screened positive for ASD and 506 screened negative for the disorder.

Secondary diagnostic testing for autism was conducted at age 21 on 70 of the children who screened positive and 119 of those who screened negative for autism.

Overall, the results showed that about 5% of the 623 low-birth-weight children in the study had ASD, compared with about 1% found by the CDC in the general population.

Low Birth Weight and Developmental Disabilities

Pinto-Martin says the autism rate may be even higher among the low-birth-weight babies born today.

At the time the babies in the study were born in the mid-1980s, a 1-pound baby that survived was considered a "miracle baby." Now babies weighing 1 pound are routinely kept alive through advances in neonatal care.

Experts say low-birth-weight babies may face a number of developmental and movement disabilities as they grow up, and autism may one of them.

"As our technology in the neonatal intensive care units is able to allow babies that are born earlier and earlier and smaller and smaller to survive, there may be other consequences of that," says Alycia Halladay, PhD, director of research for environmental sciences at Autism Speaks, an autism advocacy and science group. "That is definitely something to consider, and something that deserves further study."

Halladay says this solid study strengthens the science behind the widely held belief that low-birth-weight children are at higher risk for autism. But more research is needed to determine the nature of that relationship.

Pinto-Martin agrees.

"It may just be a marker for the real cause," Pinto-Martin tells WebMD. "Just because you have a baby born with a low birth weight doesn't mean they are at risk for autism. There may be something that happened during their neonatal course, or it may be an infection in the mother both precipitated the early delivery and the autism.

"This is where the work really begins," says Pinto-Martin. "We have a really important clue now, and we need to look at how this helps us learn about specific causes of autism."